In the past several years, much research has been conducted in the manufacture, composition, and use of crystalline shape-selective catalysts and molecular sieves, and the patent literature reflects both the economic significance of this area of research and the technical value of the prolific but apparently small increments of progress heretofore. Much of the work has concentrated on refinements of Silicalite and alumino-silicates.
Crystalline aluminosilicate "molecular sieves" can be described as rigid three dimensional networks primarily of tetrahedra of SiO.sub.4 and AlO.sub.4 in which the silicon and aluminum atoms are cross-linked by the sharing of oxygen atoms.
The basic configuration of silicon and oxygen atoms in a pure Silicalite composition is at least theoretically a lattice of tetrahedra. With the introduction of aluminum atoms to replace some of the silicon atoms, the tetrahedra containing them are considered to have negative valences and, typically, a sodium atom is formed to balance a negatively charged tetrahedron. The sodium atom, however, is not believed to be an integral part of the tetrahedral lattice structure, and in any event is amenable to ion exchange.
Prior art developments have led to the creation of many synthetic crystalline materials which are generally similar to naturally-occurring zeolites such as faujasite and mordenite. Synthetic crystalline aluminosilicates which are the most common and are described in the patent literature and publications have been designated by letters or other convenient symbols. Examples of these are Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979) and others. Other examples in the ZSM series are described in U.S. Pat. Nos. 4,016,245, 4,046,859, 4,287,166, 4,397,827, 4,448,675 and many other patents of Mobil Oil Corporation. See the article entitled "Shape-Selective Reactions with Zeolite Catalysts"; particularly part IV by Warren W. Kaeding, L. Brewster Young, and Chin-Chiun Chu, Journal of Catalysis 89, 267-273 (1984), describing the alkylation of toluene with ethylene to produce p-ethyltoluene as typical of the literature on the use of such materials as ZSM-5 in alkylation techniques.
Also previously disclosed are crystalline silica composition materials which exhibit molecular sieve properties characteristic of a number of crystalline aluminosilicates, but which exhibit none of the ion exchange properties which are requisite for a zeolitic molecular sieve. Such materials, of which a paradigm is described in a Union Carbide patent (U.S. Pat. No. 4,061,724), have been called Silicalites and are characterized by a very low aluminum content. See also U.S. Pat. Nos. 4,285,922 and 4,397,827, and Flanigen et al in Nature, V. 271, Feb. 9, 1978, p. 512. The use of aluminum-free materials is extolled in U.S. Pat. Nos. 3,941,871, 4,088,605 and 4,462,971.
Other crystalline silicates exhibiting both molecular sieve properties and ion exchange characteristics consist of three dimensional networks of SiO.sub.4 and FeO.sub.4 tetrahedra--see U.S. Pat. No. 4,208,305.
Representative of the patent literature on the use of crystalline silica, aluminosilica and similar catalysts for alkylation of aromatics are U.S. Pat. Nos. 3,751,506, 3,755,483, 4,002,698, 4,034,053, 4,086,287, 4,104,319, 4,113,788, 4,117,026, 4,127,616, 4,128,592, 4,288,647, 4,371,714, 4,158,024 and 4,447,666.
In the prior art there are references to the "promotion" of crystalline silica catalysts by the addition to preformed crystalline silica of various agents such as arsenic oxide, phosphorous oxide, magnesium oxide, boron oxide, antimony oxide, amorphous silica, alkaline earth oxides, (see U.S. Pat. Nos. 4,208,305 and 4,288,649) alkali metal carbonates and mixtures and precursors of the foregoing. In all these past teachings (i.e., Herkes U.S. Pat. No. 4,283,306, Dwyer U.S. Pat. No. 3,941,871), the promoters are added by impregnation or extended contact of a preformed crystalline silica with a liquid medium containing the additive. These techniques are familiar to persons skilled in the art and are reminiscent of those employed (see U.S. Pat. No. 3,031,420) in producing hydrotreating catalysts wherein cobalt and molybdenum solutions have been impregnated on supports such as alumina, or reforming catalysts where platinum salts have been impregnated on appropriate supports. Other catalyst preparations in which supports such as silica, alumina, clays, etc., have been promoted with various metals by a variety of methods such as impregnation, ion exchange, vapor deposition, etc., are familiar to workers in the art.
The preparation of two separate solutions to mix for forming a precipitate to be crystallized is discussed in U.S. Pat. No. 4,117,026 (see example 21 for the post-addition of magnesium), and the technique is also employed in U.S. Pat. No. 4,462,971. This is not the same as the "composited" approach mentioned for the addition of "silica-magnesia" in column 7, lines 27-40 of U.S. Pat. No. 3,702,886.
Magnesium has been added to crystalline silicates by extended contact in the aqueous phase, or by multiple impregnation, or by various other approaches to modifying a preformed silica structure. See, for example, U.S. Pat. Nos. 3,972,832, 4,034,053, 4,113,788, 4,117,024, 4,128,592, 4,137,195, 4,158,024, 4,166,047, 4,275,256, 4,283,306, 4,367,359, 4,370,508, 4,371,714, 4,371,721, 4,379,027 and 4,477,585. As will be seen below, our crystalline magnesium silicates are made in an entirely different manner, the magnesium being incorporated into the crystalline structure during its formation rather than after. There is no need to speak of the magnesium we use as a "replacing" cation as do the authors of U.S. Pat. No. 4,046,859 (col. 6, line 52). See also U.S. Pat. No. 4,200,528, which describes an amorphous magnesium silicate. The magnesium in our composition cannot be removed by conventional ion exchange techniques.
Our crystalline magnesium silicates also differ from the prior art in that by our procedure we have formed specific reaction products with characteristic X-ray diffraction patterns.